Hi,
if you don't use Projectserver there is no way to get the total of the project's cost into a formula. So you only can do a calculation in a macro like this one:
--- cut here
Sub CalculateCostPercentage()
Dim oTask As Task
For Each oTask In ActiveProject.Tasks
If Not oTask Is Nothing Then
oTask.Number1 = Format(oTask.Cost / ActiveProject.Tasks.UniqueID(0).Cost * 100, "0.0")
End If
Next
'optional
ActiveProject.Tasks.UniqueID(0).Number1 = 100
End Sub
--- cut here
Just open the VB Editor, copy the code into it and run it.
Good luck!
thausla